Image sensors with a global shutter style frame timer are designed such that pixel integration happens simultaneously for all pixels in the array. In this case, the operations associated with the beginning and end of pixel integration happen essentially simultaneously for all pixels. The operations associated with the beginning of integration (e.g., the reset operation) and with the ending of integration (snapshot operation) can have transient effects on pixel references and power supplies of both the pixel and row driver. Depending on the specific pixel design and array size, these transient demands may be severe enough that they may cause image artifacts or collapse a supply or reference altogether.
Moreover, even with basic metal signal routing, the global shutter operation does not happen for all pixels truly simultaneously. The routing induces a small time-of-flight delay such that the operations on the row closest to the signal driver will happen before those on the row furthest from the signal driver. This delay can help mitigate the previously mentioned transient demands. The time-of-flight delay, however, may be insufficient, in which case it may be desirable to insert additional row-to-row delay. Increasing the delay, however, decreases the ‘global’ nature of the shutter.
There is an optimization to find the minimum row-to-row delay that effectively will spread out transient demands on the power supplies and references to the point there are no artifacts.
Different pixel modes of operation and frame size (in the case of windowing, for example), may change the transient demands on the power supplies. There may also be some desire to trade-off minimal image artifacts for a ‘faster’ global shutter. Additionally, the array artifacts are difficult and time intensive to model and simulate.